Samsung 840, TRIM disabled for two months:
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Samsung 840, TRIM enabled again, SSD freshly trimmed using fsck command:
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Note that this is an older Samsung 840 (not "EVO"), so your experiences might be different. But I have experienced a similar speed degradation (noticeable in real life) with my previous Samsung SSD (SATA-1, don't remember the model). TRIM fixed it.
That is a drastic difference. How full is your drive?
I'm still kind of afraid to reenable it on my work laptop. It's been all around flakey recently, I'll tough it out until it's time for a replacement.