Have an experience with iijmio prepaid sim?

I have a plan to go to japan in mid november and want to stay connect with internet.so I decide to use prepaid internet sim card.
But i heard that prepaid simcard that sold at airport frequently sold out .So I decide to purchase it before hand.But I don't sure about activation period of iijmio simcard because other prepaid simcard have 14days activation period after purchase.
Anyone known about this?
Thank you.

What do you mean by "activation period"? The "tourist" prepaid SIMs from iijmio are probably mostly the same as all the others such as bmobile, econnect, etc.

others prepaid sim card like so-net said must be activate in 14 days after purchase.

https://t.iijmio.jp/en/qa/index.html#qa01

Q4 After purchase, by when should I start using the Japan Travel SIM?
A4 The initial communication must be performed by the end of the deadline month for registration for usage as described on the package.

From above, IIJMIO FAQ page that mean you should activate/setup your SIM card based on the deadline date printed on the package.

And the usage period for simcard will depend on what service/data plan you purchased. In case on IIJMIO Travel Sim
there are 2 plans,

a) 1GB or 1 month whichever come first after initial SIM activation
b) 2GB or months whichever come first after initial SIM activation

In term of speed connection, most all prepaid SIM use Docomo lines. So in theory all should be the same, but depending on how many customers are using the line, the speed may varies.

Thank you very much!!!

In term of speed connection, most all prepaid SIM use Docomo lines. So in theory all should be the same,

Actually, they use the Docomo network only for the cellular connection. Then, each provider has its own Internet infrastructure providing the actual Internet connection, and they can and do throttle bandwidth at this point (although this is more noticeable on long-term plans than on tourist ones).