slow on pcs to apply group policy

i have a 2 root DCs (1 root and additional to it 2008 enterprise x 32) & 2 child (1 child and additional to it 2012 r2)

i found it is slow on applying group policy on new pcs join domain after 3rd time restart approximately

and i found always high load on DCs in ping for example the time is 500 ms and if i open dc locally or remotely if i make r.c to press refresh it came after asecond generally not fast !! if i open task manager i found ram is 47 % and cpu 28 %

note : all pcs joined to child domain 2012 r2 and almost clients are win 7

note : no hanging

note the dcs are on pcs with specs core i5 & ram 4 giga

note : i make slow link detection to 0

note : when i find ping is 500 or more on 2 child (1 child and additional to it 2012 r2) i restsat them one by one every one restart the time ping be 1 ms !!! and no load that happened for 1 hour or more ! and then back to high load !!