Why is my gaming ping or latency high when using a travel eSIM?

Two reasons. Mobile networks generally have higher latency than home fiber, and a travel eSIM's data may route through a partner or home gateway in another country, which adds distance and ping. To reduce it, connect to strong local 4G/5G, avoid congested networks, and prefer a local country plan — local plans usually route within the country, so latency is lower than some regional or global plans.