Regretting 15.7.3 on iMac late 2013

shortly, I updated to 15.7.3 through system update thinking it will improve sluggish performance of Sequoia (mouse movement lagging, safari website pages slow and etc) . Moreover, after root patch and restarting device I get weird screen until desktop shows up.

my questions:

1- is my iMac spec not enough for Sequoia?

2- what to do to improve performance (preventing sluggish and laggging?

3- how to deal with black screen each time after restart?