Two cheap (not absolute bottom, Phison PS3110-S10, small size - 120GB, so speed is smaller) SSD drives, one is MLC (on left) and one is on TLC (on right). Sequential writing scenery.
As soon as SLC buffer in TLC disk ended write speed dropped to below HDD level.
Read speed is ok on both, nothing to worry.
Here it is similar for small one, but better for larger.
Older article on issue
And can be even worse
Speed can drop to below 50MB/sec
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