OP:
Do you have the original, factory-installed, platter-based hard drive inside?
If so,
THAT'S what is "slowing everything down".
REPLACE IT with an SSD.
It's so easy a job that ANYONE can do it --
this means YOU. (and yes, I'm pointing my finger at you)
You want a 2.5" SSD, same size (or larger) than your internal.
Many out there, I like Crucial and Sandisk.
You should also spend an extra $10 or so and get a USB3 2.5" drive enclosure, like this:
SABRENT 2.5 Inch SATA to USB 3.0 Tool Free External Hard Drive Enclosure [Optimized for SSD, Support UASP SATA III] Black (EC-UASP)
www.amazon.com
You can use the enclosure to "prep and test" the SSD BEFORE you install it, so you know beforehand that everything works.
Once installed, put the old drive into the enclosure and it will become a backup or extra storage drive.
TOOLS:
You will need a Phillips #00 driver and a TORX T-6.
These can be found at hardware stores or online.
Go to ifixit.com to see what's involved.
Again -- it's EASY.
Prep work:
Put the SSD into the enclosure
Connect to Mac and initialize/erase it with Disk Utility.
If you're using High Sierra or later, choose APFS.
If you're using Low Sierra or earlier, choose Mac OS extended with journaling enabled, GUID partition format.
Now, download CarbonCopyCloner from here:
www.bombich.com
Use CCC to "clone" the contents of the internal drive to the SSD.
It will take a little while, be patient.
When done, the SSD is an EXACT COPY of the old internal drive, but will run MUCH faster.
DO A TEST BOOT:
Reboot and hold down the option key CONTINUOUSLY
When startup manager appears, select the SSD with the pointer and hit return.
You should boot from the SSD.
Get to the finder and "look around".
If everything looks as it should, now POWER DOWN.
NOW ... do the drive swap.
When done, reboot with the "option key trick" one more time.
Select the SSD again (even though it's installed internally).
Do you get a good boot?
One more thing:
Open the startup disk preference pane and re-select the SSD to be the new boot drive.
Close startup disk and reboot one more time.
Get a "good boot"?
You're done.