Depends on the project you are working on. It could be hard and could be easy. Thicker piece of walnut You have to carve harder than it becomes. But on another hand, it all relative to the woodcarving tools you have. 

Ask those three questions :

How many wood carving tools you have?

More tools you have more You can do. At list famous woodcarver of 17th-century Grinling Gibbons thought so. When You will visit England, visit some places with the work of Grinling Gibbons. Note what year he carved and compare it with previous works of a master.

You will find a very interesting fact. Later work of GG is much more intricate. You can say it is because Grinling Gibbons accumulated experience with years of everyday carving. It is true, but only to an extend. But and big but, Grinling Gibbons himself was a collector of woodcarving tools, and if a book written by David Esterly about Grinling Gibbons is accurate. GG said it is because of the woodcarving tools he collected during his professional life as a Master woodcarver, he could carve those amazing pieces of true woodcarving art.

Is walnut hard to carve?

It is easy to carve with the right tools. The bigger the woodcarving tools You have faster you will carve your project. And easier it becomes.

How sharp Your wood carving tools are?

Sharpness of woodcarving tools is one of the most important keys to success in art of woodcarving in general. And especially it applies to carving walnut because walnut is a little harder than basswood. In my school of woodcarving I have multiple videos about how to sharpen woodcarving tools. Metter in fact my very first woodcarving lesson I recorded was on sharpening woodcarving gouge. 

Often I see frustrated beginners in woodcarving hobby because of dull tools. When You are buying woodcarving tools, usually they come pre-sharpened " ready to carve" out of the box. It it could be and it also could be bad. What I mean by that? t is good because You can pull a woodcarving gouge out of the box and start carving. But y question is: How long it will last? 30 minutes, maybe an hour? your tool will become dull quick enough. It is good that you are getting sharp tools, but it is better if You reshape and resharpen Your woodcarving tools from day one. 

For example,

What bevel it the best for woodcarving? 

Different wood, different bevel it requires for woodcarving. Softer wood, like basswood you will need low bevel. Low bevel will slice trough fibers of basswood like a knife through butter. Harder wood, like oak You will need higher bevel. Higher bevel is stronger and you need a strong edge o a gouge to carve oak. Walnut is not to soft, and it is not too hard. You will need medium bevel, more on lower side.