@bernhard @teppo @netcarver, though disabling the SessionHandlerDB module resolved the website speed issue, but that created another major problem. As my site is running on an ASG setup, there are multiple server instances running and delivering site contents at the same time. Now the session is stored in the file system, it randomly loggs-out the processwire user when switching between the instances (Switching of instances happens on its own, we can't handle it, if I turn off all the instances and keep only 1 then it will solve the problem, but will create performance issues as the website has large number of visitors).
The only possible and reliable solution is to store the sessions on the database, do ProcessWire supports this natively without using any module?