The term<em> cumbersome</em> is defined as "large or heavy and therefore difficult to carry or use". This is fitting for the first blank as the context expects a negative adjective that expresses difficulty.
The term <em>steadily </em>refers to something firmly fixed, supported, or balanced. This is fitting for the second blank as it can be used to describe a manner of walking. In this case, a<em> plod</em> (heavy) can be done steadily, but not quickly.