Is Morgan slower than Stricklen?
Probably not, but she can't hit 3's like Stricklen can. Morgan hasn't had any significant health or injury issues this year that I am aware of. I think she has been available for every game. However, since she gets fewer than 10 minutes in most games, that probably hasn't taxed her knees very much.
Actually, I disagree with psconn's assessment that she is half a step slow. Remember that she is backing up Alyssa Thomas at the 4, and "AT" is a good enough player that it would be difficult for anyone who is behind her to get a lot of minutes. I think the other difficulties that he mentioned (such as the traveling calls and inconsistent 3-point shooting) are the result of not getting enough in-game time (for understandable reasons).
I think if Morgan got anywhere near the minutes that AT has gotten, she might be equally effective (assuming that her knees could hold up), although not in the same way. AT has absolutely NO 3-point shot (not even really a 15-foot jump shot), so she has to do all her scoring a few feet from the basket, which is hard to do at 6-2, but she does it. Morgan can play in the paint as well (she rebounds at least as well as AT and perhaps better), and she also has a midrange and 3-point game which AT lacks. But she can't handle the ball nearly as well or pass as accurately, and she doesn't play with the same swagger as AT.
It's possible that AT will be one of the casualties of the salary cap and/or free agency. If that happens, I think Morgan is ready to fill her shoes in the starting lineup, and (always assuming that she remains physically OK) the Sun will be able to move into 2020 with no shortfall at that 4 position.