Something along these lines. I took out your sort and head commands, as I wasn't sure how you'd want to cut off the values. :
index=f5 instance=test earliest=-4d@d latest=-1d@d
| eval urlall=split(uri,"?")
| eval url=mvindex(urlall,0)+"*"
| eval timeFrame=if(relative_time(now(),"-2d@d")>=_time,"old","new")
| chart count avg(reqtime) as avgtime by url timeFrame
| rename "count: old" as countold "count: new" as countnew "avgtime: old" as avgtimeold "avgtime: new" as avgtimenew
| where count>100
| eval avgtimediff=avgtimenew-avgtimeold
| eval avgtimediffpercent=tostring(floor(avgtimediff*100/avgtimeold))+"%"
| eval countdiff=countnew-countold
| eval countdiffpercent=tostring(floor(countdiff*100/countold))+"%"
| table url,countold,countnew,countdiff,countdiffpercent,avgtimeold,avgtimenew,avgtimediff,avgtimediffpercent
... View more