Yes, most airline affinity credit cards have a limit to miles that can be earned per calendar year. To my knowledge, the SPG card does not. If you concentrate your spending on this card, you can do two things:
Get SPG Gold status after 30k spend
If you transfer the SPG points to AA miles, you can get AA Gold after 800k spend. There is no fee to transfer these points.
DC has a tax to convert their points to miles, as does Amex.