I see only CPU overload here...
... click on "CPU" tab (second from left), then organize the "Processes" window by "Average CPU" (larger numbers at the top).
Do some screenshot while you experience the slowdown.
Also there are some peaks on the Hard disk, so click on "Disk" tab, then on the "Disk Activity" window and arrange it by "Total (B/sec)".
(higher on the top)
Do some screenshot while you see those peaks happening.
consider your done sir....
from cpu tab
from disk tab
Please do not skip the last steps, as these screenshots are valueless...click on "CPU" tab (second from left),
then organize the "Processes" window by clicking on "Average CPU" till the larger numbers on the top.click on "Disk" tab, then
on the "Disk Activity" window and arrange it by clicking on "Total (B/sec)" till higher numbers are on the top.