How would you say something like "I'm lagging again, my ping is spiking to 500(ms)" in Putonghua and Cantonese (in the context of online games)? I've heard someone say "網又卡了！" before, but I'm not entirely sure how common that is.
Yes, in putonghua，We say "网又卡了， 我ping都500了， 卡死了（damn lagging）， 你卡不？小王怎么一点都不卡呢？”
“Ping” is totally acceptable in Putonghua， just like “windows，NPC，BOSS”， all game players know it， or you can use "延迟”， “我延迟有500了” 。
Some technical terms do not have formal translation or still used in English form for the sake of convenience.
'Ping' is one of such example, alternatively you can use latency in Chinese '延迟'.
It would be more appropriate to use 'ping' among IT guys and use latency '延迟' for non-IT users. However, in Cantonese especially among people from Hong Kong, it's advised to use English when referring to technical terms as the translation can be very different from Mandarin, unless you are very sure of the correct translation.