1) Does it gives significant speed boost ??
It's not the purpose of TRIM to give a speed boost. The purpose of TRIM is to maintain the drive's Read/Write performance.
2) Is it compulsory to have TRIM in SSD??
Modern SSDs don't rely only on TRIM to maintain drive performance; they also use Garbage Collection to maintain performance. So no, TRIM is not compulsory.
3) Will my SSD not slow down even when it's almost full?? since old/useless block are replaced by new data instantly instead of removing them first then writing data on those blocks
Yes, most SSDs will slow down when they are more than 90 percent full.