Clocking in is pretty slow in version 9.6 when the item has a large

[Eli Qian]

I use the following method to get the time taken for `org-clocl-in`:
(benchmark 1 '(org-clock-sum (org-clock-get-sum-start)))

In my case, the item has 1400+ entries in its logbook,  clocking in will cost
1-2s to finish(benchmark results: "Elapsed time: 1.689801s (0.527506s in 7
GCs)"). That's abnormal because it is done very quickly in version
9.5.5.(benchmark result: "Elapsed time: 0.057270s")

After investigation by using profiler, I found the problem is caused by
`org-element-at-point`, which was added to `org-clock-sum` in version 9.6.

Any idea about speeding up clocking in?