Since you already have Scannerz, there's a tool in that package that I've grown fond of called Performance Probe. It can do load averaging. Some people are having MDS problems on Yosemite, and MDS is a complete wart hog.
To find out if it's the problem, do this:
Here's the catch: If you see processes that contain the letters or variations of letters like "mds", "mdworker", or "mds_stores" consistently running with CPU levels in the top 5 or 10 it or with CPU percentages actually registering CPU usage more than say 0.2% means that the meta data server probably has problems with the index and it's continually indexing. This set of processes, mds and all the little mdworker threads it launches are REALLY super CPU and drive intensive.
If you open up Activity Monitor and look at the processes sorted by CPU you'll probably see the mdworkers keep popping up at really high levels but only for a second or two. Then some will disappear and others will pop up.
The difference is that Performance Probe is averaging them over 100 seconds (that's what the 100 count was), so if they're showing up at all and they're constantly present, it means they're very highly active. If you only see them crop up once in a while, then it's normal. In fact, on a stable system that's really not being loaded, the biggest CPU consumer should be Performance Probe itself and it should only be a few percentages.
If the mds process and it's threads are the problem you'll need to use mdutil to delete and rebuild the index files. When that happens the system WILL bog down, but it should complete and be stable, usually in an hour or so. It depends on what data you have. If mds can't make sense of some data on your drive, that might be the problem and it may never settle down.
Also if mds is NOT the problem and there's some other app that's constantly using high CPU levels, maybe by something that's installed on the unit, it will also tell you what that process is because it will be at or near the top of the list. Finally look at the regular display for Performance Probe and see the indicators. On a system that's idle and stable the bars for I/O, Memory, and CPU will all be green, and the memory pie chart should show a reasonable amount of free memory. The memory pie chart is similar in appearance to the old versions of Activity Monitor but it isn't because it has compressed memory and all that other stuff accounted for.
Another thing that also occurred to me is this: You said you had an external drive connected to the system. Does it have drivers? If the drivers or the drive itself aren't fully compatible with Yosemite and this bottleneck is appearing when you access files or open up Finder, it will cause a delay waiting for the attached unit to spin up if it's asleep. Bad drivers, if installed, may make the thing just lock up until the OS can resolve the problems. I don't know if that would show up in Performance Probe, it might. I'm thinking that sort of incompatibility would mean the OS is blocking waiting for drive access, making you think there's something wrong with the system. When there isn't.
To find out if it's the problem, do this:
- If you have anything else running that you know of, terminate it
- Start Performance Probe
- Click on the Start button
- Click on the Advanced Monitoring button
- When the heads up display pops up, in the lower right corner uncheck the Hide When Unfocused button (Optional, but it will keep the HUD on top)
- Click on the Load Monitoring tab
- At the bottom there are 2 sliders. Slide the top slider to 100, then the bottom slider to 100
- Click on the Start Monitoring button
- Wait 100 seconds
Here's the catch: If you see processes that contain the letters or variations of letters like "mds", "mdworker", or "mds_stores" consistently running with CPU levels in the top 5 or 10 it or with CPU percentages actually registering CPU usage more than say 0.2% means that the meta data server probably has problems with the index and it's continually indexing. This set of processes, mds and all the little mdworker threads it launches are REALLY super CPU and drive intensive.
If you open up Activity Monitor and look at the processes sorted by CPU you'll probably see the mdworkers keep popping up at really high levels but only for a second or two. Then some will disappear and others will pop up.
The difference is that Performance Probe is averaging them over 100 seconds (that's what the 100 count was), so if they're showing up at all and they're constantly present, it means they're very highly active. If you only see them crop up once in a while, then it's normal. In fact, on a stable system that's really not being loaded, the biggest CPU consumer should be Performance Probe itself and it should only be a few percentages.
If the mds process and it's threads are the problem you'll need to use mdutil to delete and rebuild the index files. When that happens the system WILL bog down, but it should complete and be stable, usually in an hour or so. It depends on what data you have. If mds can't make sense of some data on your drive, that might be the problem and it may never settle down.
Also if mds is NOT the problem and there's some other app that's constantly using high CPU levels, maybe by something that's installed on the unit, it will also tell you what that process is because it will be at or near the top of the list. Finally look at the regular display for Performance Probe and see the indicators. On a system that's idle and stable the bars for I/O, Memory, and CPU will all be green, and the memory pie chart should show a reasonable amount of free memory. The memory pie chart is similar in appearance to the old versions of Activity Monitor but it isn't because it has compressed memory and all that other stuff accounted for.
Another thing that also occurred to me is this: You said you had an external drive connected to the system. Does it have drivers? If the drivers or the drive itself aren't fully compatible with Yosemite and this bottleneck is appearing when you access files or open up Finder, it will cause a delay waiting for the attached unit to spin up if it's asleep. Bad drivers, if installed, may make the thing just lock up until the OS can resolve the problems. I don't know if that would show up in Performance Probe, it might. I'm thinking that sort of incompatibility would mean the OS is blocking waiting for drive access, making you think there's something wrong with the system. When there isn't.